214 



ZONES AND REGIONS 



[Pt. Ill, Sect. I 



season in tropical continental districts, and must be of considerable im- 

 portance in relation to geographical botany ; it is well known that in 

 Bengal thin sheets of water freeze during the night in the dry season. 

 In districts on the coast, in the forest, and on the hills the cooling induced 

 at night-time by radiation is much less, thanks to the large amount of 

 water-vapour in the air, although physiologically it is by no means in- 

 significant. 



The following table gives a summary of mean temperatures in parallel 

 belts of the tropical zone : — 



MEAN TEMPERATURE IN PARALLEL BELTS OF THE TROPICAL 



ZONE 1 (after Spitaler). 



iii. LIGHT AND THE ULTRA-VIOLET RAYS. 



The intensity of the rays of light, like that of the heat-rays, is naturally 

 greater within than beyond the tropics, and the tropical daylight is 

 brighter than that of the temperate or polar zones. This feature may 

 be directly observed in the clearer reflection from the surface of water 

 and also from that of foliage when wet with rain, and it is very noticeable 

 in photographs taken in the tropics. True as this is of the luminous rays, 

 it is equally true of the chemically effective ultra-violet part of the 

 spectrum. 



In the tropical districts of America and of the Asiatic islands that I know, the 

 intensity of light is however not so strong as in East Africa, regarding the conditions 

 of light of which P. Reichard writes as follows : ' In the natural features around us, 

 the dazzling brightness of the air is most striking. The more vertical tropical sun 

 produces a much brighter light than in Europe. At first the eyes can barely endure 

 the glare, so that one is obliged to wear blue spectacles and to draw one's hat 

 down over one's eyes 2 .' 



There are no direct comparative observations regarding the intensity 

 of luminous radiation for different latitudes, but some observations have 

 been made regarding the chemical rays. Thus simultaneous observa- 

 tions at Kew and at Para in Brazil on three April days of the year 1 866 

 showed an intensity of chemical action nearly twenty times greater at Para 

 than at Kew ; even in August it was 3-3 times greater at Para. 



1 Harm, op. cit., Bd. II, p. 17. 



2 Deutsche Rundschau, Oct. 1894. Quoted by Hann, op. cit., Bd. II, p. 40. 



